State officials issued an air pollution alert for much of Minnesota on Friday after the week’s unusual weather created a “temperature inversion” that trapped increasing levels of airborne contaminants.

The Minnesota Pollution Control Agency (MPCA) urged people who are sensitive to poor air quality — including those with cardiovascular or respiratory problems — to avoid strenuous outdoor activity.

In the Twin Cities, the alert was expected to expire Friday evening as winds pushed the smoggy air east.

State officials attributed the alert to the same weather system that brought rising temperatures to the state throughout the week. In this case, the alert didn’t mean more pollution had been released into the air, simply that existing pollutants couldn’t cycle out normally, according to MPCA analyst Cassie McMahon.

Under normal weather conditions, the air tends to be warmer near the ground, while cold air is higher up in the atmosphere. Since warm air rises, this creates a natural cycle that washes pollutants out of the air.

But when a wintertime warm front moves in, it often travels over the top of colder air near the ground, which is still chilled by snow cover. The warm air acts like a lid on top of the cold air, trapping pollutants in place.

The trapped pollutants include fine particles from motor vehicle exhaust, wood burning stoves and fires and other energy-burning sources, plus byproducts from chemical reactions that occur in the air, the agency said.

In addition, McMahon said, the recent snow melt has released more moisture into the air, which can create fog that further holds the pollutants in place and provides more surface area for chemical reactions to occur. And since this week’s warm front came from the south, McMahon said, the warm air passed through more industrial areas than Minnesota’s usual westerly or northern wind, so that pollution mixed with the typical local pollutants.

Unhealthy pollution levels began Thursday afternoon in the western half of the state and were expected to return to normal statewide by Friday evening.

This was the year’s first air quality alert, but Minnesota typically has four or five per winter, and more are possible as the weather warms, officials said.

See the latest air quality conditions at


Rebecca Harrington is a University of Minnesota student reporter on assignment for the Star Tribune.